This invention relates to lasers.
More particularly, the present invention relates to lasers that generate relatively long wavelengths.
Vertical cavity surface emitting lasers (hereinafter referred to as xe2x80x9cVSCEL""sxe2x80x9d) include first and second distributed Bragg reflectors (hereinafter referred to as xe2x80x9cDBR""sxe2x80x9d) formed on opposite sides of an active area. The VCSEL can be driven or pumped electrically by forcing current through the active area or optically by supplying light of a desired frequency to the active area. Typically, DBR""s or mirror stacks are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. In conventional VCSEL""s, conventional material systems perform adequately.
However, new products are being developed requiring VCSEL""s which emit light having a long wavelength. The indium gallium arsenic nitride (herein after referred to as xe2x80x9cInGaAsNxe2x80x9d) material system can be used to fabricate light sources that have been shown to emit at wavelengths near 1300 nm, and up to 1500 nm for some edge emitting devices. These wavelengths are important for fiber optic communication systems. The InGaAsN material system can be closely lattice matched to gallium arsenide (hereinafter referred to as xe2x80x9cGaAsxe2x80x9d) and, therefore, can be epitaxially grown on a GaAs substrate. This feature is significant because gallium arsenide/aluminum arsenide (GaAs/AlAs) DBR materials can then be used to form the highly reflective mirrors of a 1300 nm long wavelength vertical cavity surface emitting laser (hereinafter referred to as xe2x80x9cVCSELxe2x80x9d) with InGaAsN or InGaAsN grown with the addition of antimony (Sb) (hereinafter referred to as xe2x80x9cInGaAsN:Sbxe2x80x9d) quantum wells as the active gain medium.
However, a problem encountered in making long wavelength VCSEL""s based on the InGaAsN material system is the difficulty of incorporating a sufficient mole fraction of nitrogen into the lattice of the grown epilayers in order to reduce the bandgap for long wavelength operation. The nitrogen incorporation can be improved by reducing the epitaxial growth temperature for both plasma assisted molecular beam epitaxy (hereinafter referred to as xe2x80x9cMBExe2x80x9d) and metalorganic chemical vapor phase deposition (hereinafter referred to as xe2x80x9cMOCVDxe2x80x9d), with a resultant degradation in material quality and photoluminescence intensity. Also, the nitrogen mole fraction can also be increased much more substantially by introducing a plasma source in an ultra-high vacuum plasma assisted MBE chamber to produce more nitrogen radicals.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser.
It is an object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser which incorporates more nitrogen into the active region.
It is another object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser which has an improved radiation efficiency.
It is still another object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser which has high quality distributed Bragg reflector mirrors.
It is still another object of the present invention to provide a new and improved electrically pumped long wavelength vertical cavity surface emitting laser which can be fabricated with a higher growth rate.
It is a further object of the present invention to provide a new and improved electrically pumped long wavelength vertical cavity surface emitting laser which has distributed Bragg reflectors that have continuously graded heterointerfaces.
To achieve the objects and advantages specified above and others, a method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser is disclosed. The method includes epitaxially growing a distributed Bragg reflector positioned on a compatible substrate wherein the distributed Bragg reflector is epitaxially grown using MOCVD. A GaAs cladding layer is epitaxially grown on the distributed Bragg reflector and an active region is epitaxially grown on the cladding layer wherein the active region is epitaxially grown using plasma assisted MBE. A cladding layer is epitaxially grown on the active region and a distributed Bragg reflector is epitaxially grown on the cladding layer wherein the distributed Bragg reflector is epitaxially grown using MOCVD.
In the preferred embodiment, the substrate includes GaAs and is oriented in the (100), (111), (211), (311), (411), or another off-axis crystallographic direction. By orienting the substrate off-axis, the radiation efficiency is improved. Also, the light emission can be confined to a desired region by using lateral oxidation or proton implantation. Further, the light emission can be confined to a desired region by forming a mesa in the distributed Bragg reflector.
One reason MOCVD is used to epitaxially grown the distributed Bragg reflectors is because it is easier to control the doping and the interfacial compositional grading. Also, the growth rate using MOCVD is faster than plasma assisted MBE which is important because DBR""s are relatively thick.
One reason plasma assisted MBE is used to epitaxially grow the active region is because it is easier to incorporate nitrogen into the semiconductor layers to reduce the bandgap for long wavelength operation. Thus, the method of fabricating the electrically pumped long wavelength vertical cavity surface emitting laser combines the advantages of both the plasma assisted MBE and MOCVD growth techniques.